The present invention relates to a video camera such as a VTR integrated camera to and from which a DC power source light is mountable and dismountable respectively for illuminating an object of which image is to be picked up.
When an image of an object is to be picked up at night or in an environment of darkness where there is no sufficient brightness, an illumination unit is used to illuminate the object to acquire sufficient brightness. As an illumination unit to be used for a video camera, a DC power source light which is mountable and dismountable to and from the video camera is known. An exclusive battery pack is mountably and dismountable incorporated in the DC power source light. When a switch of the DC power source light is turned on, the DC power source light emits light to illuminate an object.
Such a conventional DC power source light, however, has a problem that since an exclusive battery is incorporated in the DC power source light, the light itself becomes large and very heavy, so that when this is loaded on a video camera to pick up images, the video camera as a whole becomes large and very heavy which is difficult to handle.
Further, in the above-described conventional DC power source light, no consideration is given to the case where the capacity of the battery becomes small.
For example, in the case where a camera user has forgotten to turn off the switch of the light to disconnect the light from the power source after he or she used the DC power source light to emit light for a prescribed illumination time, there is a possibility that the battery continues discharging until its capacity has been completely wasted. A battery currently available in the market comprises a plurality of cells, and when a discharging continues as described above an imbalance may occur in the state of changing to cells of the battery. When the magnitude of this imbalance becomes large to cause large differences of voltages among the cells, an output voltage of a cell having a high voltage is forcibly applied to a cell of a lower voltage at its both terminals. If such a state lasts, the battery is deteriorated. For example, in the case where a nickel cadmium battery is used as a battery, when an output voltage of a certain cell is applied to another cell because of an imbalance of the state of changing to the cells as described above, hydrogen may be generated in the battery to deteriorate the battery which may incur a dangerous state. This problem will similarly occur in the case where a plurality of batteries are used in parallel, as well.
Further, in the above-described conventional DC power source light, no consideration is given to excess current which occurs at a point of time when the power source is connected by turning on the switch.
In other words, the resistance temperature characteristics of the filament of a lamp fitted to a DC power source light is set such that when the filament temperature is high, the filament resistance takes a value to maintain the filament current at a rated value, but when the filament temperature is low, the filament resistance becomes close to zero. Therefore, at the moment when the switch is turned on to connect the power source, the filament resistance takes a value close to zero because the filament temperature is low that time, so that a current which exceeds a rated reference value flows to the filament, and accordingly, to the battery. As a result, the life of the battery is lost and a power source noise is mixed into other portions of the video camera.